Add `<{edit,edit-or-view,view}-raw-message>` functions

Add `<{edit,edit-or-view,view}-raw-message>` functions

Add mutt-functions `<edit-raw-message>`, `<edit-or-view-raw-message>`
and `<view-raw-message>`.

`<view-raw-message>` is a binding for the C-function
`mutt_view_message()`, which opens the raw message as a read-only file
in an external editor.

`<edit-or-view-raw-message>` is a dispatcher function: If the mailbox
is writable then this is the same as `<edit-raw-message>`, otherwise it
is the same as `<view-raw-message>`.

`<edit-raw-message>` is a synonym for `<edit>` whose name coherent with
the naming scheme of the other two functions introduced.

Change default key binding of `e` from `edit` to
`edit-or-view-raw-message`. This allows viewing the raw message in an
external editor also in read-only mailboxes.