Implement subscription feeds

[str4d]

This is a tracking ticket for work on subscription feeds in the branch changeset:h:i2p.i2p.str4d.test - specifically, for tracking the implementation of the base feed structure and mechanism, and its application to the /home apps list.

[str4d]

The current implementation (changeset:c023e50f4b0cd1bdbe90e4b8add52841e125771d) is functioning - a test feed can be successfully added and apps within it can be both rendered on /home and hidden via /confighome. Feeds can also be updated manually through the routerconsole, as well as deleted, and there is a CLI for signing and verifying feed XML.

Next issues:

• FeedFetcher is using Preferences.importPreferences() which does not perform the feed validation. When swapped to use SignedPreferences.importPreferences() the feed data is not accessible, despite the importer completing with no errors. Adding a sync() call after the import doesn't appear to help.
• There is something odd happening with the category view. On first view of /home after a restart, the single category I have is visible, but when I click into it (and see the category contents as expected), subsequent visits to /home do not show the category, and additionally the category's "hidden" column on /confighome is permanently ticked (and the category appears in the manual feed as well).

[str4d]

changeset:0e14c324ad842724a29cc1e213d90a6650912680 fixes the first bug above, and changeset:d36c86970a12c04711302f78f7c049a645c85bae fixes the second. Another bug has appeared now, though (after fixing the second bug above): apps in feeds cannot be unhidden. They can be added to the hide list, but removal fails. The hide list seems to be populated correctly in ConfigHomeHandler, so the bug *should* be in HomeHelper.hideApps().

[str4d]

changeset:924d3f1e0b49e213901fbfd80a1a19093bcc0544 fixes the app unhiding bug.

[str4d]

On hold, I need to merge the branch with trunk, and rethink the design.

[zzz]

We abandoned this approach. We implemented Atom (XML) news feeds, wrapped in su3 files, in late 2014 (0.9.17) to mid-2015. We aren't using Preferences classes, we soured on them. If we did want to implement feeds for the /home apps list, we'd want to start over and base it on what we did for news, i.e. the current "base feed structure and mechanism".