Reword login text

[rfc2822]

Purpose

It was not clear what "Add account" does, especially that it doesn't just sign in to an app-proprietary hosted account like for 99.9% of the apps. This was also the reason that Google had sometimes rejected the app because they didn't "get a test account on our server".

We

• split Welcome and the welcome text for easier translations,
• make it clear that it's about a user server → empty accounts text, privacy note
• decided to keep the words
  • Add account for the whole process of adding an account and
  • Login for the first step of this process
• changed the last button to Finish to distinguish between the title of the whole process (Add account) and the specific step.

Short description

• Change empty accounts text
• Add privacy note to Login screen
• Changed last button text

Checklist

• The PR has a proper title, description and label.
• I have self-reviewed the PR.
• I have added documentation to complex functions and functions that can be used by other modules.
• I have added reasonable tests or consciously decided to not add tests.

[sunkup]

Works 👍