Use new installer feature from packager.io

Postinstall for openproject is now in its own addon at https://github.com/pkgr/addon-openproject.
Add addon-legacy-installer
move watchers group into work package group

- watchers group did mirror implementation
- work package group contained watchers PATCH action that is not
available as specified

Specification and implementation for watchers should now be in sync
again
add already implemented links to WP-Spec
add already implemented links to Project-Spec
add already implemented property to User-Spec
add specification for existing Status-Endpoint
condense introduction

- use fewer groups (making sidebar navigation in rendered Specification
more useful)
- remove redundant error object chapter
add specification for existing Priorities-Endpoint
add specification for existing Categories-Endpoint
add specification for existing Versions-Endpoint
allow flash messages to be permanent
when a query cannot be loaded, let angular display an error message
add compatibility disclaimer to intro
add 'show' action to QueryPolicy
return a 404 when accessing a not-viewable query via the experimental API
renaming queries renames it's menu item
simplify index action on exp.api work_packages_controller
Merge pull request #2035 from opf/fix/apiv3_watchers_spec_outdated

[DOC] Update outdated specification on APIv3 watchers
Show unsupported browser notice for Firefox 10-30

We only support Firefox >= 31 (latest ESR) now. As we previously
supported older Firefox versions, especially 24, we show an
unsupported browser notice for them now.