Reorganize subscription management code

There is still a lot of work to do. Some links are broken.

See https://github.com/marienfressinaud/FreshRSS/issues/646
Add selected feed on main subscription page

- When an id is given, corresponding feed is displayed
- Refactor code
- Improve coding style

See https://github.com/marienfressinaud/FreshRSS/issues/646