Add manual trigger to JenkinsCI.
This change adds the ability for Integrations to run JenkinsCI manually.

A checkbox is added to the JenkinsCI form that indicates if JenkinsCI should

only run manually.
The password form label has also been updated to emphasize using an API token

instead of the user's password since newer versions of Jenkins only works with

API tokens.
This change also fixes the Jenkins API failing for Review Board 3 because

request.open() was not valid.
Based on work by David Trowbridge: /r/11130

Manually tested on Review Board 3. Jenkins was set up with Docker and

connected to a GitHub repository and Review Board 3. A Jenkins build was

not run when the manually run checkbox is unchecked and is run when the

field is checked. Clicking on the 'Run' button runs the Jenkins build.
Verified the disabled/enabled run manually field by testing on Review Board

3.0.18, 3.0.19 and 4.0.x.
Verified retry functionality when state is in ERROR and TIMEOUT states.
Unit tests to verify that a build is not run when the manually run

configuration is checked and to verify that a build is run when the signal is

emitted.