Update 2.8.1 release notes

- Add fixes that were merged since the last update.

- Add missing issue IDs.

- Improve formatting.

Change-Id: Ic14711754309b25577a157cf4deba99d05950d51
Bump VERSION to 2.8.1

Change-Id: I1ea1c84a6f3d7deb6dfee977551a18b2c46acb37
Update Gerrit API version to 2.8.1 in pom files

Update the version in the GWT plugin API and plugin
archetypes.

Change-Id: I6dad48410f5ac1ab8e5de4977e8acc0766cb0445