[iOS] Merge feed management Interests and Following

This CL removes the previous "Following" entry in the feed management menu, along with the "Follow management" component that it would open. Now, the previous "Interests" entry has been renamed to "Following", and it retains its old behavior which would just link to the interest manager webpage.

Thus, most of this CL is just removing files and strings, and then renaming "Interests" to "Following" in several components.

Before: http://screen/BPgN6af28gz3Jja
After: http://screen/uZdTk7eQ2MbiQth

Change-Id: Ic6af1810336be13d531c246245a5e83cc522d5fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5608255


Commit-Queue: Adam Arcaro <adamta@google.com>
Reviewed-by: Ginny Huang <ginnyhuang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312844}