Final polish on filter and sorting interactions

[miketaylr]

One final bug before the issues page is "feature complete" ☑️

A few things we need to do to make the interaction between Filter buttons and Sort dropdowns:

• make it possible to pass in existing params to get_search_results (otherwise you get a results/UI mismatch)
• normalize between search and Issues API differences (possibly disabling some things where it isn't possible):
• The search API uses a "sort" param, while the Issues API uses "direction", etc.
• Some of the Issues params need to be added to q, like the mentioned param turns into a mentions token in q. Will figure out the mapping in a comment.

[miketaylr]

Issues API params we use:

param	values
state	open, all
creator	username
mentioned	username
per_page	25, 50, 100

sort states	combined values
"newest"	sort=created&direction=desc
"oldest"	sort=created&direction=asc
"most commented"	sort=comments&direction=desc
"least commented"	sort=comments&direction=asc
"recently updated"	sort=updated&direction=desc
"least recently updated"	sort=updated&direction=asc

Issues to Search API conversion:

old param/state	conversion
state	into q as "state:open", "state:closed"
creator	into q as "author:username"
mentioned	into q as "mentions:username"
"newest"	sort=created&order=asc
"oldest"	sort=created&order=desc
"most commented"	sort=comments&order=desc
"least commented"	sort=comments&order=asc
"recently updated"	sort=updated&order=desc
"least recently updated"	sort=updated&order=asc

note: per_page: stays the same