Update copy of sign in button when loading
Update copy of sign in button when loading

- when loading, updated "Sign in" copy to "Signing in"
- checked with speech synthesis, no issue
- added a comment for translators

CP-1546

Co-authored-by: Nicolas Hoffmann <3e093dc5a9b4d587679f9cea8032a1e3ce9160f6@gmail.com>
Co-authored-by: mmso <f03b14a56c033f2ed01e3f21eab07a99813cb7c2@protonmail.com>

ProtonMail/WebClient,ProtonMail/WebClient,ProtonMail/WebClient