Fix bug when emptying a category

- Controller expected a POST action but we were using a link (so a GET action)
- Links to empty a category is now a button whit a formaction
- Improve comparaison id with id of the default category
Bug feed name with special chars

https://github.com/marienfressinaud/FreshRSS/issues/462
Minz: remove Minz_Cache

https://github.com/marienfressinaud/FreshRSS/issues/163#issuecomment-37990319
https://github.com/marienfressinaud/FreshRSS/issues/303
Merge remote-tracking branch 'origin/dev' into 163-export