Consider adding a warning when starting Icedove with Claws Mail persistence feature

According to the migration instructions, people should not start Icedove before doing migrating their data. We could add a warning when starting Icedove if the Claws Mail persistence feature is enabled and point to the migration instructions.

The warning could be:

The <em>Claws Mail</em> persistence feature is activated.

If you have emails saved in <em>Claws Mail</em>, you should [migrate your data](https://tails.boum.org/doc/anonymous_internet/claws_mail_to_icedove] before starting <em>Icedove</em>.

[Start <em>Icedove</em> anyway] [Exit]

Feature Branch: feature/10458-icedove-warn-if-claws-mail-persistence-enabled

Parent Task: #7773 (closed)

Original created by @sajolida on 10458 (Redmine)