Small cleaning of the code

* Move large portions of code from `inc/feeds.inc.php` to `inc/install.inc.php and index.php`
* Use `collate nocase` on unique text fields (tag name and url)
* Add a bunch of comments
* Put some TODOs with some ideas (not necessary with high priority)
* Enhance `install()` function (databases creation, `data/` dir creation)
Merge master branch