[IMP] website_slides: perform some code linting and re-order frontend templates

Purpose is to perform some linting in code. No functional change should
occur with this commit, only some code move. Diff may appear huge but
it is just because some files are split.

This cleaning allow to better understand code and templates architecture
now that eLearning feature has landed in Odoo Community. It will ease future
additions, modifications and fixes.

Containing :
 * [REF] make some internal slide-related methods private as they are not
   intended to be called by outside world;
 * [MOV] new channel invite wizard in wizard folder;
 * [MOV] split frontend templates in some files to better locate code. There is
   now files for homepage, course, lesson, embed and utils in addition to
   slide-related profile template;
 * [REM] dead (unused) templates;

Commit linked to task ID 1941250 and PR #31272.