Introduce ViewModel for RssFeed*Activity

Closes #1894 (closed)

Testing instructions

This MR refactors the whole RSS Feed system.

Access RSS Feeds-screen in the "action overflow" (3 dots-icon) in the app bar of the Blogs activity.

It is at all times during testing -- like after actions taken, events occuring etc -- relevant to rotate the device and ensure that:

• Text entered in fields, switches toggled etc remain so
• Visibility of progressbars and buttons are the same
• Labels on buttons remain the same
• Opened dialog boxes remain so

Import a feed

• Tap Plus-icon in the app bar
• Enter a feed URL
  • example valid feed: https://briarproject.org/news/index.xml
  • invalid feed (a HTML page): https://briarproject.org
• Tap import
• Wait for import to succeed, and to be returned to RSS Feeds-screen.
  • or expect import-failed dialog, if feed was invalid
• Verify that successfully added feed appears in list on RSS Feeds-screen.

Be impatient, after "Tap import" above:

• Return to previous screen ◁
• Wait for successfully added feed to appear in list
  • Or, in the case of an invalid feed, expect import-failed dialog.

Be even more impatient, after "Tap import" above:

• Return to previous screen ◁
• Tap Plus-icon to add a new feed while the first one is being processed
• Expect the progressbar to be visible
• Expect to be returned to RSS Feeds-screen once ongoing import is successful