Related T1900

This function will get called a lot (by default, up to 70 times
per origin search), so it deserves a much higher rate-limit.

It will replace the 'internal' endpoint _origin_latest_snapshot,
and won't require fetching the whole snapshot just to know
if there is a visit with a snapshot.

It will be removed in the next release of swh-storage (v0.0.146).

Related T1768

Test for navigation buttons on error page

Split tests into different contexts

Related T1768

these constants should be set in the SWH_CONFIG_FILENAME at deployment,
they have no reason to live in the code.

this is needed so we can spawn a production-like webapp (in docker)
without a memcache (makes documentation writing easier).

'revision log' endpoints with complex lookups are excluded
from this commit, as it's impossible to get a non-ambiguous URL
for them without excessive hacks.

Related T1768

It makes debugging easier.

swh-sidebar-collapsed value in localStorage due to other tests fails these tests.

It exist cases where file badly detected input encoding as ascii which results
in display errors in browse.