Implemented support for build logs

Summary:
Depends on D7519.

This implements support for build logs in Harbormaster.  This includes support for appending to a log from the "Run Remote Command" build step.

It also adds the ability to cancel builds.

Currently the build view page doesn't update the logs live; I'm sure this can be achieved with Javelin, but I don't have enough experience with Javelin to actually make it poll from updates to content in the background.

{F79151}

{F79153}

{F79150}

{F79152}

Test Plan:
Tested this by setting up SSH on a Windows machine and using a Remote Command configured with:

```
C:\Windows\system32\cmd.exe /C cd C:\Build && mkdir Build_${timestamp} && cd Build_${timestamp} && git clone --recursive https://github.com/hach-que/Tychaia.git && cd Tychaia && Protobuild.exe && C:\Windows\Microsoft.NET\Framework\v4.0.30319\MSBuild.exe Tychaia.Windows.sln
```

and observed the output of the build stream from the Windows machine into Phabricator.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7521