Milestone/Iteration equals list implies OR, not AND

If you perform a GLQL query like milestone = ("17.5", "Backlog"), the resulting table will include issues with either of the two milestones since milestones cannot be ANDed.

This is different from assignees or labels where = implies AND.

Solution

A query like milestone = ("17.5", "Backlog") should throw an error and milestones should support in operator to query issues by multiple milestones.

Notes:

• This also applies to iterations, when they will be implemented.
• There is missing test coverage for milestone related queries in the compiler's specs.
• The != operator should continue to work as usual because it implies not either of/not or.

cc @johnhope