Allow use of "OR" operator in MR search
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### Proposal
The current MR search allows you to specify a list of filters that are *anded* together. This syntax is insufficient to be able to express the following useful filter:
* Show me all the MRs where I need to take action.
This might be expressed as `(Reviewer = self AND NOT Approved-by = self) OR (Author = self AND state = opened)`
I note that the current syntax for the `NOT approved-by = self` is `Approved-by != self` which is strange as `Approved-by` is actually a list and so you would expect Approved-by!=self to be true if a reviewer other than you had approved the MR.
issue