Add support for hashtag on user interface

Add migration to fix typo
Add support for linking hashtags with posts
Rework tag search page so it says a nicer message than page not found
when no post use that tag
Add new string to translation

migrations/postgres/2018-10-20-164036_fix_hastag_typo/down.sql

@@ -0,0 +1,2 @@
+-- This file should undo anything in `up.sql`
+ALTER TABLE tags RENAME COLUMN is_hashtag TO is_hastag;

migrations/postgres/2018-10-20-164036_fix_hastag_typo/up.sql

@@ -0,0 +1 @@
+ALTER TABLE tags RENAME COLUMN is_hastag TO is_hashtag;