Introduce update channels

The idea is to be able to choose a channel from a predefined list.
Instead of having the beta checkbox option to enable the beta versions, we will be able to select either release, beta, alpha or whatever we would like to add in the future.

The maintenance cost will be next to zero if we want to add a new channel then.

Remove the beta-update-site-url Option

We currently have 2 options to set updates website URL: --beta-update-site-url and --update-site-url. Since the refactoring of the updater and the (not so) new update server structure (NXDRIVE-1143), and as we wan to add new channels, we decided to drop the --beta-update-site-url option.

The only way to set a custom update site URL is now the --update-site-url option.

Add the New channel Option

We will have more than a dual options to choose (release/beta), so it makes sens to let the user choose what channel to use by adding a new option: channel. Its default value is release.

This option replaces beta_channel which will be removed too.

GUI side, the beta checkbox is replaced by a dropdown menu with:

• release (default)
• beta
• alpha

Retrocompatibility

We need to take care of current user setting about the beta channel. In the database, we should remove the beta_channel and add the channel one with the good value.