LibreSignage v1.1.0 release

After a lot of development LibreSignage v1.1.0 is finally
here. This release includes a ton of new features as well as
bug fixes and more. Users of LibreSignage v1.0.x are encouraged
to upgrade as soon as possible.

This release includes support for automatically migrating data
created by older versions of LibreSignage so the upgrade process
should be quite easy. There's no manual work needed; data is
migrated automatically when the web interface is accessed.

Some major changes:
  - A rewrite of the server-side API system.
  - Major backend code improvements.
  - Support for HTTP caching slides resources.
  - A data migration system for simplifying upgrades
    between LibreSignage versions.

Docker images for this release will appear in the LibreSignage
Docker Hub repository as "eerotal/libresignage:v1.1.0" and
"eerotal/libresignage:latest". You can also build this release
from source by following the instructions provided in the readme.
Note that the build system has changed a bit from previous versions,
so you should probably give the instructions a read even if you
have built previous versions yourself.

Changelog v1.1.0:
  - [internal] Separate private and public files in the directory tree
               to naturally prevent access to private files. (PR #56)
  - [feature]  Implement [bgimg] and [bgcolor] markup tags for styling
               slide backgrounds. (issue #75)
  - [feature]  Implement buttons for manual slide navigation in display.
  - [internal] Mostly rewrite the API system
  - [internal] Use PHP autoloading and namespaces instead of direct
               file includes.
  - [internal] Improve various classes and PHP files to make them more object
               oriented.
  - [internal] Implement a testing framework with PHPUnit and write tests
               for all API endpoints.
  - [bugfix]   Fix various bugs in the backend that were discovered while
               writing tests.
  - [build]    Make the build system more modular.
  - [build]    Write separate interactive and CLI versions of the apache2-debian
               target. (apache2-debian-interactive and apache2-debian)
  - [build]    Various improvements to the build system.
  - [docs]     Generate documentation with Doxygen.
  - [internal] Implement HTTP cache support for slide asset fetching endpoints.
               (issue #53)
  - [bugfix]   Fix a bug where uploading/removing slide assets was allowed even
               when the slide was not locked. (issue #81)
  - [bugfix]   Fix a bug where a slide could be deleted even when it was
               not locked. (issue #80)
  - [bugfix]   Fix a bug where an error was thrown when changing queues if an
               unsaved slide was open. (#86)
  - [feature]  Implement a data migration system for simplifying upgrades
               between different versions of LibreSignage.
  - [bugfix]   Fix outdated information in documentation.

Huge thanks to everyone who helped the development by contributing code,
submitting issues or giving feedback.
This tag has no release notes.