[FIX] mass_mailing: increase email clients compatibility

Creating a mail template which is complex and compatible with all mail
clients is a very difficult task as each mail client supports a
different subset of basic CSS rules.

This commit fixes 3 problems which were occuring in outlook and/or
thunderbird but not in gmail:

- Main layout horizontal overflow
    -> change 24px horizontal padding of main layout to 3% and set the
       width to 94% (so that the sum is 100%)

- Fixed 600px content was full width instead
    -> Change the .o_mail_wrapper to be a <table> element instead of a
       <div> element. The table is full-width and contains one row of 3
       cells: an empty left one, a fixed 600px-width center one and an
       empty right one. This allows to have the same layout as before but
       without using the unsupported max-width css rule.

- Texts were not using the right color and font-size
    -> <p> elements were not properly inheriting the color and font-size
       properties, so force the style on <p> elements.

opw-743156