SideBySide2: Expand margin between paragraphs in comments

Users have noted there is insufficient space between paragraphs of
text in comments in SideBySide2.  Increase the margin to 0.5em by
adding 0.2em at the start of each paragraph.  This has the effect of
increasing the spacing between the user name and the first paragraph.

Change-Id: I729583388131ff14eeb248d410217f42cfe0d977
Bump servlet API to 8.0.5

Change-Id: Ic812ac12efb4a7189b1bd9827a353051b7f14211
Tomcat: Describe configuration to allow embedded slashes

Tomcat takes the extra precaution of disallowing embedded slashes
by default. This breaks Gerrit urls.

Describe configuration option how to tweak Tomcat.

Bug: Issue 2600
Change-Id: I1448b1e3f0234eb7993dc223c3c273cfe188d669
Add field _number to ChangeInfo

This field is also available in ChangeJson.ChangeInfo.

Change-Id: Iaa4c7c5b1e7c9089a8ad5f30b629fa19e8982279
Merge changes I1448b1e3,Ic812ac12

* changes:
  Tomcat: Describe configuration to allow embedded slashes
  Bump servlet API to 8.0.5
Merge "SideBySide2: Expand margin between paragraphs in comments"
Add default empty implementations for extensions-api interfaces

This way it's easier to add new api without breaking existing impls.
Default implementation throws a NotImplementedException.

Change-Id: I19e495745baa8d628cab861a3ac95acaf7de3836
Merge "Add default empty implementations for extensions-api interfaces"
Add hint how to use IntelliJ IDEA for Gerrit development

Change-Id: I4a8b6fbf432d253189c0ca5c5d7b8730871e2712
SideBySide2: Add Shift-a keybinding to show/hide left side

When a new file is added there is nothing to display on the left side.
Some reviewers may prefer to fill the screen with the right side while
reading the new code.  Add a setting to toggle the visibility of the
left side, and a keyboard binding of "Shift-a" to trigger this.

The preference is not currently saved to prevent reviewers from
missing deletions in modified files.

Change-Id: Ic6659de6bdfb8b564a2cbcb2cc8ccab871af9e6a
Merge "SideBySide2: Add Shift-a keybinding to show/hide left side"
Merge "Add hint how to use IntelliJ IDEA for Gerrit development"