Add glyphs.Replacement model

[kwiliarty]

• [x] foreign key to map
• [x] ascii
• [x] foreign key to glyph
• [x] unique on ascii, mapping, glyph
• [x] set up many-to-many fields with through on glyph and mapping
• [x] natural keys and get_by_natural_key
• [x] ascii is a reserved word, rename
• [x] tests
• [x] gw- aliases
• [x] load
• [x] dump
• [x] migration loading
• [x] rename map to mapping?
• [x] longest_first to order by length(ascii)
• [x] add to admin
• [x] populate
• [x] replacements unique on text and mapping