Various fixes for `global-discussion-list-filters` feature #1926

Fixes https://github.com/sindresorhus/refined-github/issues/1919

Links to reproduce: global Pull Requests or Issues, then click either the Commented or Yours button.

Things this PR fixes:

1. In the global Issues page, the proper parameter is put in the URL so the filter is correct (i.e. pull requests don't appear as well).

2. When in a Commented or Yours query, the added query parameter is removed correctly from the rest of the buttons URLs (Created, Assigned, Mentioned and Review requests). The previous behaviors removed one space too many leading to parameters joined together. Fixes #1919.

3. When the query parameters contained a malformed query, the Commented or Yours buttons don't light up. For example, if the query contains commented:username-and-something-else, the regex doesn't match now, as opposed to it matching previously and highlighting the Commented button.

4. In the global Pull Requests page, the Commented and Yours buttons also include in the query the parameter is:pr, as was done for is:issue (fix 1) in the Issues page.

These fixes are listed in the order of the commits.

cc @bfred-it