Projects: remake issues tab as project detail

[chadoh]

fixes #1865

Rather than having an Issues tab at the top that, when visited, shows all issues for all projects, you can now only see issues for one project at a time.

This has a few benefits:

• A simpler filtering experience: three filters instead of four.
• Less wasted vertical space: the filter bar now contains a back button, and there's no need to show tabs. Your list of issues displays closer to the top of the page.
• Code maintainability: expect faster changes to projects, now that we were able to simplify the complex logic resulting from multi-project filtering.

I recommend viewing this diff with indentation changes hidden

Screenshot

Three quirks to note:

1. When clicking through from a project to an issue, the heading of the page disappears momentarily – this would be awkward to code around, and I don't think it's worth it. The reason this happens is because it is coded in a way that keeps the URL minimal (project detail at /projects/:projectId, issue detail at /issues/:issueId), and ensures that refreshing the page will still load data correctly and show the correct heading

2. When you apply filters, then select an issue, then click "Back", your filters are lost – this has two causes: 1. We are not currently implementing filtering via the URL, 2. Even if we were, aragonAPI does not support true "go back" behavior; we could hack something together ourselves, but it would work imperfectly

3. The "Show More" behavior has not changed – you'll see how clumsy it is for large projects in the screen recording below

[chadoh]

@ottodevs good points on the performance issues. I've added two refactor commits here that address it somewhat, but it seems that the biggest reason the performance has regressed is the inclusion of react-axe. If you comment out the block that adds it in index.js, the performance now seems to match the performance on Rinkeby & mainnet.