Explain policy exception rules to users

Summary:
Ref T603. Adds clarifying text which expands on policies and explains exceptions and rules. The goal is to provide an easy way for users to learn about special policy rules, like "task owners can always see a task".

This presentation might be a little aggressive. That's probably OK as we introduce policies, but something a little more tempered might be better down the road.

Test Plan: See screenshot.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7150
Provide better strings in policy errors and exceptions

Summary:
Ref T603. This could probably use a little more polish, but improve the quality of policy error messages.

  - Provide as much detail as possible.
  - Fix all the strings for i18n.
  - Explain special rules to the user.
  - Allow indirect policy filters to raise policy exceptions instead of 404s.

Test Plan: See screenshots.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7151
Make more Diffusion controllers/views capability-sensitive

Summary:
Ref T603. I got most of this earlier, but finish it up.

  - Make a couple of controllers public; pretty much everything in Diffusion has implicit policy checks as a result of building a `DiffusionRequest`.
  - Add an "Edit" capability to commits.
  - Swap out the comment thing for commits.
  - Disable actions if the user can't take them.

Test Plan: Viewed a bunch of interfaces while logged out, got appropriate results or roadblocks.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7152
Show "Search" in menubar while logged out if users can access it

Summary:
Ref T603. If an install allows acccess by logged-out users, show search.

(A lot of the search typeahead results, although visible to the user, don't lead anywhere interesting right now. We can clean this up in the future.)

Test Plan: As a logged out user, searched for some stuff. It worked. Also, I only found results I could see, which is quite heartening.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7153
Move policy config to "Policy" app and make `policy.allow-public` description scarier

Summary: Ref T603. We have a real policy app now, so put the config options there. Revise the description of the public policy switch to make it clear that enabling it immediately opens up the user directory and various other interfaces.

Test Plan: Viewed/edited config setting.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7154
Tweak static resource package definitions

Summary: Add a couple more resources that we need on most pages.

Test Plan: Regenerated resources, viewed homepage.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7155
Support audio files with HTML5 `<audio />`

Summary: Ref T3887. Similar to how we render images with `<img />`, render audio with `<audio />` if possible.

Test Plan: See screenshots.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3887

Differential Revision: https://secure.phabricator.com/D7156
Make <audio /> work slightly better on devices

Summary: Ref T3887. `300px` is a little too wide on devices.

Test Plan: Viewed on a phone.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T3887

Differential Revision: https://secure.phabricator.com/D7157