feat: store images on peertube servers

[julien-rg]

Standards checklist:

• The PR title is descriptive.
• The PR doesn't replicate another PR which is already open.
• The PR has self-explained commits history.
• The code is mine or it's from somewhere with an Apache-2.0 compatible license.
• The code is efficient, to the best of my ability, and does not waste computer resources.
• The code is stable and I have tested it myself, to the best of my abilities.
• If the code introduces new classes, methods, I provide a valid use case for all of them.

Changes:

When uploading an image, now tries to upload on a Peertube server.
If it does not work, it fallbacks to Imgur.
During app initialization, a random Peertube is chosen from the list of servers (returned by the "roys" proxy call).

Other comments:

Currently, the Peertube authentication is used with username and password: "test_bastyon" (see app.js / line 124).
So we need to make sure each Peertube server has this account created with the correct password.

[shpingalet007]

Consider naming your PR branches with better name next time. Using master branch for pushing pulls is complicating the work with the git branches tree.