Use information from DOM in `clean-issue-filters` #2926

• Milestones count is always available in the DOM. We should never ask the API

<img width="377" alt="Screen Shot 2020-03-24 at 01 32 08" src="https://user-images.githubusercontent.com/1402241/77375950-4be02680-6d6f-11ea-8b28-f732fb541f45.png">

• Projects might be available, look for this info before querying the API

https://github.com/sindresorhus/refined-github/blob/c84162f1dc7ad898c7a3e303da285004c8c996da/source/features/remove-projects-tab.tsx#L64

• Change the cache to only store "has-projects:${getRepoURL()}": boolean, we don't need the actual number

Since these 2 are now independent, this should probably look like:

``` js function init(): void { hideMilestones(); hideProjects(); }