[FIX] mail: replace special characters

When the delimiter '@' is used in the composer, a regular expression is
built in order to suggest followers. However, if this regular expression
is incorrect, it causes a crash since RegExp will throw a syntax error.
Since a new regular expression is built for each new character the user
types, this situation can occur very easily. For example, typing '@('
will cause a crash.

This fix replaces characters which may easily lead to a syntax error by
a space.