Add a multi-select project filter to issue and MR lists

Problem

There are three 'types' of page where you can see issues or MRs from multiple projects: the dashboard, the todos page, and the group-level pages.

Of these, only the todos page has a project filter, and even then, it doesn't allow for selecting multiple projects.

Once we have the new search bar available on these pages, we should allow filtering by project.

Original description

The following step after https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8519 has been merged is to include:

• On the project filter, to have the projects you actually have todo's for to be on top
• And have an indicator, how many todo's you have for those, something like:

cc: @oswaldo @smcgivern @gsmethells