[IMP] doc: indexify index

* opt-in toc navification
* unhide index toctree, it's now back to a list of documents
* remove text thing
[IMP] doc: more indexy index
[IMP] doc: move old API sections to orm toplevel

also move porting checklist/doc/thing to bottom of document, it's mostly
for seasoned or internal developers
[IMP] doc: try to make some bits of the porting guide less confusing
[FIX] tests: make self.assertRaises() return the expected object
[IMP] doc: rename WS page, lift to top-level
[IMP] doc: wording of method-calling introduction
[IMP] doc: have ws page take whole page width

Doc pages used container() which is width-limited, works acceptably for
single-column content but WS page takes 2 columns with code, having as
much space as possible is a much better idea.

Mayhaps it should even use lg-column(2) instead of md-column(3)?
[IMP] doc: improve index, WS page
[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
[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] website_sale: product website description must be translatable
[FIX] account_budget: theoretical amount: ignore future and use datetime
[ADD] account_budget: theoretical amount: tests
[FIX] account_payment: fixed the view of payment order to display the amount of transactions
[IMP] account_payment: remove dead voucher code and extension hook

Since bank statement do not use vouchers anymore in 8.0,
it's not necessary to create the voucher anymore.

Additionally, I add an extension point to let modules adapt
the statement line.