Marker: support duplicate marker type on single portal

[le-jeu]

This was a limitation in early version of the server that was lifted some versions ago. This is different from multi-assignments.

Use cases

• Meet up at different deltas at the same portal (or farm/upgrade)
• Destroy/Capture/Virus/Repeat
• Multiple Other markers
• User imagination

[cloudkucooland]

No objection. The initial restriction was due to a client-side issue. We made it impossible to add multiples at the server to get-around the client problem. There is no reason to keep this restriction in place now.

[bgillock]

This would amount to removing the code that disables the existing marker types in the select list. I suggest doing this before #309 as it makes the bulk logic simpler.