Remove one block type relationship from issue links

Issues can block others with two relationship types, { TYPE_BLOCKS => 1, TYPE_IS_BLOCKED_BY => 2}:

source_issue	block_type	target_issue
issue_1	TYPE_BLOCKS	issue_2
issue_3	TYPE_IS_BLOCKED_BY	issue_1

On the example above issue_1 is blocking two other issues using two different types of block.

The extra relation type adds complexity when querying for blocking/blocked issues as can be seen on IssueLink class.

If possible we should consider using just one kind of blocking type to make queries simpler and a bit faster. We will still be able to identify if an issue is blocked or blocking using the target_id and source_id keys.

This task will need migrations. We can get rid of the relation that has less records on database, which is this case is TYPE_BLOCKS => 1 We can migrate records with link_type = 2 to have link_type = 1 and stop saving records with link_type = 2:

gitlabhq_production=> SELECT COUNT(*) FROM issue_links WHERE link_type = 1 \g
 count 
-------
 10410
(1 row)

gitlabhq_production=> SELECT COUNT(*) FROM issue_links WHERE link_type = 2 \g
 count 
-------
 26218
(1 row)

For more context check the discussion on !34665 (comment 365697703)