Use LONGTEXT for templates in MySQL

TEXT in PostgreSQL is an unlimited column size. However, in MySQL, it is
limited to 2^16 bytes. LONGTEXT allows 2^32 bytes, which is overkill,
but will prevent MySQL from silently truncating this column and thus
rendering the entire tenant's API access moot.

Fixes bug #1210799

Change-Id: Ib2b379db32a8296cf8c5a2434e7615afc59288e3