Year in music: Add playlists

[mayhem]

This PR is based off @amCap1712's year-in-music branch and should first be merged into that branch!

This PR adds support for taking playlist excerpt dumps created by troi and import them into the LB database. Each dump should be formatted as follows:

<user_name>\n
<playlist_mbid>\n
<playlist_JSPF>\n

To import the file use:

manage.py dump import_yim_playlists

[pep8speaks]

Hello @mayhem! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file listenbrainz/db/dump_manager.py:

Line 324:1: E302 expected 2 blank lines, found 1

• In the file listenbrainz/db/year_in_music.py:

Line 154:50: E201 whitespace after '{'
Line 154:69: E201 whitespace after '{'
Line 154:105: E202 whitespace before '}'
Line 169:1: W391 blank line at end of file