Save the PR list filter settings.

[grokys]

Somewhere along the line, the call to SaveSettings in UpdateFilter got removed, causing the filter settings not to get persisted. This reinstates that call.

Fixes #1494

[shana]

Looks like it disappeared on commit b929d55, which is a merge commit (PR #1312)

For reference, a good way to find stuff like this is to do git log -G [regex/string to search] -m --patch, it'll output merges that have the regex or string.