Update version to 1.8.6 and add changelog informations

Version 1.8.5

Release version 1.8.4

Release version 1.8.3

Release version 1.8.0

Release version 1.8.0-beta.1

The previous revision on master (d967914), that claimed to be 1.8.0, had a
problem (see ether#3654), and so 1.8.0 was put on hold.

In ether#3660 I decided to do a beta.1 release, in order to be able to catch similar
problems.

Closes ether#3658.

Release version 1.7.5

UPGRADE NOTES: if you have custom files in src/static/custom, save them
somewhere else, revert the directory contents, update to Etherpad 1.7.5, and
finally put them back in their new location, uder src/static/skins/no-skin.

Release version 1.7.0

From now on, releases will be cut from develop, and merged directly into master.

Each release will be a tag on the master branch (e.g. 1.7.0).
A "release/1.7.0" branch will eventually be created only if/when a hotfix will
be needed.

Merge new release into master branch