Turn AbstractAddress.search_text into a TextField

@hkage raised a good point in #1384
that concatenating the address lines might lead to
a string that's longer than the set limit of 1000 characters. If I
recall correctly, a CharField limit much higher than 255 characters is
troublesome with some databases anyway, so I've taken the opportunity to
turn it into a TextField instead.

Fixes #1384.
Fixes #1385.