Closes T1067

Closes T1065

When one wants to display a content, HTTP requests are sent to the indexer
storage in order to retrieve the associated mime type, language and license.
Those requests may fail so properly handle errors in order to avoid
content display errors.

Closes T1060

This commit removes the collapsible panel, located in the top
part of numerous views, used to display the metadata associated
to a swh object in favor of a popover reachable from the Actions
menu located on the right of the view.

Related T1052

When a README is written in reStructuredText (rst), convert it to html server
side through the use of pypandoc in order to get a proper display of it.

Indeed, the client side library used to render markdown (showdown.js) does
not handle rst properly and there is no viable solution to convert rst
to html directly in the browser.

Related T1061

  - prefer to display a smaller spinner in the left of the search panel
    header instead of a big one at the center of the screen

  - clean the search results table when performing a new search

Closes T1056

This commit adds query parameters to some context indedependent views
in order to add origin info.

Closes T1055

Closes T1054

Value was turned to True in commit rDWAPPS0bc0934bca8ee36bcb1e2f8ef57c951fd5c6acca
so turn it back to False.

Closes T1053

Summary:
The tests now depend on webpack being built, so do that before running make
test.

The devserver depends on migrations being run, so do that as well.

I'm not entirely sure the migration part is the right thing to do, but at least
with the webpack part, running the tests will do reasonable things.

Test Plan: make test still works

Reviewers: anlambert, #reviewers!

Differential Revision: https://forge.softwareheritage.org/D320

  - Add a button in the top left part of origin context views to easily
    reach visits reporting view

  - Add a button in the origin visits reporting view to easily browse
    the latest snapshot

This allows to leverage client level cache efficiently and invalidate
already cached assets when source have changed.

When there is only one full visit, the last full visit link
was not displayed.

Url of type /browse/origin/(origin_type)/url/(origin_url) now redirects
to the display of the latest archived snapshot for the given origin
instead of its visits reporting.

Origin visits reporting are now available through the following url
/browse/origin/(origin_type)/url/(origin_url)/visits or by clicking in
the topmost link when browsing the archive in an origin context.

Closes T1049

This commit replaces navigation through tabs for the browse main menu
(search/help/vault/browse) by assigning a dedicated page and url to
each menu entry.

This prevents interface flashes when loading any browse page as
previously the search interface was always displayed first before
switching to the requested tab.

This commit fixes bad license name in some Python files (swh-web is
AGPL not GPL) but also adds license header in static assets related
source files (html, js, css).

As Bootstrap 3.x is no longer maintained, migrate swh-web frontend to
the last major version: 4.x (current version is 4.1).

This induces the following changes:

  * migrate from 3.x css classes to 4.x css classes

  * leverage Boostrap sass variables overriding in order to generate
    custom css for swh-web

  * use the free AdminLTE Bootstrap theme for its clean design
    (see https://github.com/almasaeed2010/AdminLTE/tree/v3-dev)

In adition to that migration process, some cleanup and prettifying
have been performed on Django HTML templates.

Related: T1032