Enforce tag length limits

[Meow]

Fixes #668

Set to a fairly generous limit of 256 bytes. Subject to change.

Querying Derpibooru database reveals that 102 tags are over the 256 byte length. Looking at which ones exactly, I can confidently say, none of them are going to be missed.

EDIT: also fixed a bug where autocomplete binary was incorrectly capped to 254 byte instead of 255 byte tag names.

[mdashlw]

There was an idea before to make it 255 bytes specifically. Any reason against it?

[Meow]

@mdashlw 256 seemed like a nice round number

[mdashlw]

@mdashlw 256 seemed like a nice round number

The justification was that 255 bytes is what the binary format of local autocomplete supports max:

philomena/lib/philomena/autocomplete/generator.ex

Lines 138 to 142 in 82a1381

	# Names longer than 255 bytes do not fit and will break parsing.
	# Sort is done in the application to avoid collation.
	tags =
	LocalAutocomplete.get_tags(@top_tags)
	|> Enum.filter(&(byte_size(&1.name) < 255))

Note the byte vs character distinction too