Add top-level Git mailmap to normalize commit author variants

This PR adds a top-level mapping file allowing to normalize commits from the same contributor and associate all commits to a canonical "First Name Last Name " author line.

The following rules are used for the construction of the .mailmap file:

for all contributors who are employees of the University or Dundee, Glencoe Software, the @dundee.ac.uk, or @glencoesoftware.com email address is used as the canonical email address
for all contributors with an executed CLA, the real name and email address sent via the CLA is used as the canonical address
for all contributors, the name used in https://www.openmicroscopy.org/teams/ or https://www.openmicroscopy.org/contributors/ is used as the canonical real name.

The .mailmap is constructed according to the official documentation using one of the two forms:

Proper Name <proper@email.xx> <commit@email.xx>

for mapping different email addresses and

Proper Name <proper@email.xx> Commit Name <commit@email.xx>

for mapping different real names.

The unique list of commit authors can be generated and reviewing using git shortlog -se

ome / omero-insight

Add top-level Git mailmap to normalize commit author variants #366