Fix bug when emptying a category
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

romibi/FreshRSS,FreshRSS/FreshRSS,marienfressinaud/FreshRSS,plopoyop/FreshRSS,superboum/FreshRSS,superboum/FreshRSS,Alkarex/FreshRSS,FreshRSS/FreshRSS,asmundg/FreshRSS,Dolomich/FreshRSS,samuelstein/FreshRSS,FreshRSS/FreshRSS,Alkarex/FreshRSS,asmundg/FreshRSS,Alkarex/FreshRSS,FreshRSS/FreshRSS,samuelstein/FreshRSS,marienfressinaud/FreshRSS,aledeg/FreshRSS,romibi/FreshRSS,Dolomich/FreshRSS,romibi/FreshRSS,Dolomich/FreshRSS,marienfressinaud/FreshRSS,aledeg/FreshRSS,superboum/FreshRSS,aledeg/FreshRSS,plopoyop/FreshRSS,aledeg/FreshRSS,Alkarex/FreshRSS,marienfressinaud/FreshRSS,romibi/FreshRSS,asmundg/FreshRSS,samuelstein/FreshRSS