Add RSS / Atom feed support

[akaleeroy]

Adding support for RSS would be very convenient as you can plug these feed URLs into feed readers to get notifications. For instance I use Riot.im (which has RSS / Atom integration).

[jhillyerd]

Interesting idea. Potential downside is for many feed readers to work, you'd have to expose Inbuckets UI to the internet.

For now, this will be blocked on the UI revamp as I will refactoring the UI controller code as part of that project.