Render the mailbody using the full width of the terminal

[romanofski]

Is your feature request related to a problem? Please describe.
With the feature for #279 (see PR #338 ) we introduced a fixed text width. The fixed width is needed in order to know the absolute position the UI needs to scroll to, since the scrolling is line based. Furthermore, the absolute width of the terminal is only known inside the rendering context of a widget.

Describe the solution you'd like
I'm proposing to create a new widget which can render our MailBody AST to the full width of the terminal. That means the text needs to be re-wrapped according to the available width of the rendering context.

Describe alternatives you've considered
None

Additional context
If the text is re-wrapped to the full rendering context's width, the absolute scrolling could obviously be off by one or two lines. Perhaps it's something which outweighs the rendering result using the full terminal width.