[FIX] website_forum - replace texttext by select2 for the tags fields
[FIX] website_forum: add fake tour for people waiting the tour. To remove in master. Without it, localstorage loop to search this unexisting tour after the install...
[FIX] doc: latex building

* add latex support for exercise admonition
* latex freezes/crashes on {HEAVY WIDE-HEADED RIGHTWARDS ARROW} so
  replace them by more manual arrows
* add some preamble configuration, may not even be necessary
* generate WS setup code only in HTML output. WS doc in latex still
  isn't great as it displays all 4 languages one after the other,
  ideally they should be tagged or something, so only one language at a
  time is generated in non-HTML outputs
[MERGE] forward port of branch 8.0 up to 5ed0739
[FIX] web_tip: prevent useless calls to eval_tip
[FIX] web_tip: properly remove the tip popover
[FIX] doc: double period in footer
[FIX] doc: incorrect class access in new api guide

When creating a class with the new API, we should use models.Model
instead of just Model.

closes #3713
[FIX] website_forum: use common method to compute tags for orm
[FIX] im_chat : momentjs undefined on external pages. Temporary fix by replacing moment() by window.moment(). The moment helper is defined in core.js, which is not imported in web_assets common. If refactoring, the dates will be pre-process in render_messages().
[MERGE] forward port of branch 8.0 up to fc329b6