Allow defining a Tpay channel ID in Pay By Link

[coldic3]

If Tpay Channel ID defined, the Pay By Link payment type will automatically redirect to the specified channel without asking a customer to choose a channel manually.

TODO:

• Refactor the JS code and move it to a .js file.
• Fix missing translations.
• Move the channel ID (e.g. for Twisto) to the dev env var and use it in the fixtures.
• Tests.
• UPGRADE file

Actions to take right after a releasing a new version:

• Update wiki/1.-Installation with customizations in templates/bundles and index.js.
• Update wiki/2.-Configuring-a-new-payment-method-in-Admin-Panel with an info about Channel ID that can be configured with PBL.