LibreSignage v0.2.0

This is the first non-beta release of LibreSignage. This release
contains a huge amount of changes, improvements and new features.
There have been ~180 commits since the last release. The changelog
is included below and it can also be found in the file changelog.txt
in the source root.

Changelog:

v0.2.0
  - [feature]  Implement keyboard shortcuts for the editor.
  - [feature]  Make it possible to preserve instance data when
               reinstalling.
  - [feature]  Implement a live slide preview in the editor.
  - [internal] Self host font files. (I actually don't particularly
               like including binary files in the GIT tree but I don't
               know of a better way so the font files are now included
               in the tree.)
  - [internal] Use SCSS instead of plain CSS for the stylesheets.
  - [internal] Import Bootstrap straight into the _default.scss file
               instead of including it in the HTML <head> tag.
  - [internal] Process CSS using PostCSS /w Autoprefixer.
  - [bugfix]   Fix a major security issue where the contents of the
               'data/' directory were accessible by requesting the
               different files with eg. a web browser. Even directory
               listings were enabled.
  - [feature]  Completely rewrite the markup transpiler system to
               improve its error detecting and reporting capabilities.
               The rewrite also introduced many other improvements. The
               transpiler now uses the familiar lexer-parser-evaluator
               architecture that most compilers use.
  - [feature]  Improve markup error reporting in the editor UI. Erroneous
               code lines are now highlighted in the editor input.
  - [bugfix]   Confirm before changing the queue if an unsaved slide
               is being edited in the editor (#17).
  - [bugfix]   Prevent creating duplicate queues in the editor.
  - [bugfix]   Don't show unstyled slide previews to users in the editor
               by making sure the necessary CSS is loaded before showing
               content.
  - [bugfix]   Fix a bug resulting in spurious slide changes on the
               display page.
  - [improve]  Make the login and user manager pages more mobile friendly.
  - [feature]  Implement a Quick Help view in the editor.
  - [Internal] Implement a new way of exporting PHP object data by using
               a class called Exportable.
  - [feature]  Implement slide locking to prevent simultaneous editing
               of slides (issue #18).
  - [internal] Rewrite most of the PHP session handling code. Implement
               a new class called Session for handling & storing session
               data.
  - [internal] Implement a simple assertion system in JavaScript.
  - [internal] Implement file uploading via the API system.
  - [feature]  Implement media storage & uploading in the backend and
               editor.
  - [feature]  Implement embedding video in slides using [video] tags.
This tag has no release notes.