Add an icon+background selector for project images

Summary: Makes it easy to choose distinctive icons for projects.

Test Plan:
{F71018}

{F71020}

{F71019}

{F71021}

Reviewers: btrahan, chad

Reviewed By: chad

CC: chad, aran

Differential Revision: https://secure.phabricator.com/D7333
Add some missing non-context translations

Summary: We're missing some strings here.

Test Plan: {F71857}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7340
Distinguish between invalid/broken handles and filtered handles

Summary:
Ref T603. Currently, we render handles the user doesn't have permission to see in a manner identical to handles that don't exist. This is confusing, and not required by policies (which restrict content, but permit knowledge that an object exists).

Instead, render them in different styles. Bad/invalid objects look like:

  Unknown Object (Task)

Restricted objects look like:

  [o] Restricted Task

...where `[o]` is the padlock icon.

Test Plan:
{F71100}

{F71101}

It's possible this renders weird somewhere, but I wasn't immediately able to find any issues. Yell if you see something.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7334
Fix lightbox downloads for embeded images and a warning

Summary:
I refactored this recently and accidentally dropped the download URI.

Also fix a warning with, e.g., files named `README`.

Test Plan: Clicked a thumb, clicked "Download", got a file.

Reviewers: chad, btrahan, dctrwatson

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D7341