Previously the app used "writeas-gtk" as the app id. This was invalid
according to freedesktop specifications.

See https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-id-generic
for details on what an App ID should be.

Creates a flatpak manifest to build the app.
Does not yet build the writeas-cli.

Related to T663

These are the directories flatpak can create while building
an app. Since they should not be committed, they belong
in the gitignore.

Flatpak doesn't support running extra commands, so we need
to make the cli build it's own target. This is made optional
since the cli doesn't always need to be shipped with the app.

Related to T663

This was missed during the consolidation of repositiories.
Since the app has different executable names for the
different builds, the app_id cannot be used as the "exec"
line unconditionally.

This run_target no longer exists.

Fixes publishing and copies the post link to the clipboard.

Allows for flatpak users to have persistent drafts,
and for the bundled cli to access them.

Required for saving to work within flatpaks. Does not affect
non-flatpak usage.

Related to T663

Needed to make sure we can create, read, and write to this
directory.

Sets ups build stage for flatpak builds.

For T678

Travis defaults to Ruby if a language is not specified,
which results in a build failure.

For T678

Docker is required for the flatpak build.

For T678

Based on the tips from travis-ci/travis-ci#2646

For T678

This ensures that Travis doesn't think we're using Ruby.

For T678

Ensure that the flatpak directories exist.

For T678

Ensures that the docker container will be able to access the
repository on the host.

For T678

Carries over the original configuration.

For T678

Required to specify a host directory.

For T678

fusermount not being executable can cause build failures
for flatpak builds.

For T678

Needed for Fuse to work for flatpak.

For T678

Incorrectly spelled a flag.

For T678

This ensures that the CI is building from the current PR
instead of git master.

For T678

flatpak-builder needs the path to the manifest.
To avoid duplication we use an env variable.

For T678