issueCount doesn't work on "non-standard" GH issue URLs

[aph3rson]

Right now, the issueCount function relies on the link attribute for each project. However, many of these links aren't in a standard format, and can't guarantee that additional options are being passed in this URL (both of which cause issueCount to not return a count).

Perhaps it might be better to update the function to compose its own URL to check against the API, by using the site and up-for-grabs/name attributes - that way, projects can link to a custom filter if need be, but the website will still render a count for viewers (albeit possibly inaccurately).

Related to #605.

[aph3rson]

GitHub's Search API allows for 10x as many unauthenticated requests, as well as supporting searches for multiple labels (incl. negation), organizations, and exposing a single count in the JSON response (without dealing with pagination - returns up to 1000 responses in one query). It might be prudent to move to using this Search API, instead of using the current Issues API.

edit: x-ref #567 (comment)