Use built-in send_error and send_response methods
Problem is that send_error(401) does not include the WWW-Authenticate header that prompts client for basic credentials. (No evident way to include required realm.) So, although it is nice and concise to use these built-ins, reverting to or writing new error handlers may be required.
Draft framework for responding to various queries
Remove superflous get() expressions
Partly hooked up FF addon panel interface
Clicking addon button displays panel with list of options - currently just choices to send all/current FF tabs to iCloud Tabs or Reading List. Queries rlictd for current iCloud Tabs on display, but doesn't insert in panel HTML yet. (rlictd isn't returning actual tabs yet anyway, just example placeholder - still need to set up icloudtabsreader/readinglistreader submodules)