[book] improve a bit the documentation of the database setup also remove the MySQL section.

[doc] add documentation for specializing translation of other cubes in an application cube

[doc] fix some URLs (cs to test jenkins autobuild )

[uicfg] Fix typo in set_fields_order

[forms] We don't want cwjs-edition-cancel class on reledit's cancel button Else it's bound to a js function that collides with onclick provided by reledit code. Closes #13120795

[test] Prefer unittest.main

[test] py.test related fix We ought not import a concrete test case from a sibling module, else it ends up partially initialized at some point and later raise assertion error in setUpClass.

[dataimport] add a bit of extra-documentation on the massive store about prepare_insert_relation limitation on inlined relation and lack of full-text indexation.

[dataimport] no need to call .keys()

[dataimport] drop massive store's flush_metadata method This method handle a temporary table which sounds useless, and even buggy in some cases (metadata for a single entity types will be only flushed once even if flush is called several time). Instead, simply call the method-doing-the job after entities have been inserted (and make it private along the way).