opt-in to remembering password

[seansfkelley]

Use a "remember me" checkbox. Tweak the phrasing/behavior of "Test Connection and Save", since it'll behave more like a normal login page.

On browser session start:

• If the user opens the popup first, show a password prompt dialog.
• If the user opens the settings page first, highlight the password field in some fashion.
• If the user uses the context menu or clicks a magnet link first, use an error notification (maybe same code path as ConnectionFailure?).

Internally, preserve the auto-relogin behavior, but keep the password in memory.

This may have overlap with #7, since they both deal with temporary/in-memory authentication.

[seansfkelley]

Implemented in the commits up to an including 90bb4ea833084adafe7083a4d6077b8adeef1a56.