Merging and splitting of creators

[anaximander23]

Currently:

• The first time a creator is encountered, their username on the source site being submitted is used as their "name"
• A previously-unknown source is assumed to be the same entity as an existing creator if their username matches an existing creator's name

So, if a mini by a Thingiverse user called "dutchmogul" is submitted, then we add a Creator whose Name is "dutchmogul". If a Patreon user called "dutchmogul" is later added, then we assume that this is the same Creator. If a Patreon user called "illgottengames" is added, then we assume that this is a different Creator.

Proposed:

• There should be a moderator tool for merging Creators with different usernames into one Creator record
• There should be a moderator tool to change a Creator's primary Name by selecting from one of the various sources they appear on, or (possibly) by directly typing in an override.
• There should be a moderator tool for splitting a single Creator into multiple records, and choosing which of the known sources belong to which, in order to resolve incorrect merges.

[anaximander23]

Looks like we've got some creators that have the same name with differing capitalisation. This feature would let us clean that up, but some smarter automatic matching would help too.