browse: Add new side tab containing the Citations Web UI

In order to integrate the Software Citations Web UI (as discussed f2f with @rdicosmo , @moranegg and @hjonin) add a new side tab on the right of the Web UI whose handle is below the Permalinks tab.

When clicking on any of the tab handles, both tabs are slided to the left. The tab whose the handle was clicked becomes the one currently displayed.

When clicking on the tab handle of a not visible tab, it becomes the tab currently displayed while clicking on the tab handle of a visible tab closes the tabs (same behavior as before).

The Citations tab is quite similar to the Permalinks one and enables to generate software citations in BibTex format when selecting an object type, the SWHID of the selected object is then inserted in the citation. To be noted, citations are generated on the fly through Web API calls. A link to browse the intrinsic metadata used to generate the citation is also offered plus the ability to copy the citation string to clipboard.

For the case where software citation cannot be generated as required intrinsic metadata are missing, an error message is displayed instead.

This is still a WIP, base logic for the tabs UI has been implemented, remains to write cypress tests and update guided tour to integrate a switch to the citation tab (in order to describe the Citations UI in a later commit).

The picked colors and icon for the Citations tab are also not definitive and could be changed.

This new Citations Web UI and its sidetab are not integrated by default and can be activated through configuration by setting the activate_citations_ui entry to True.

Once this MR accepted and merged, next step would be to enable the Citations Web UI on staging for testing purposes by end users.

Depends on !1334 (merged).

Related to product-management/swh-archive-website#4.