Add top-level Git mailmap to normalize commit author variants

[sbesson]

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 See also https://github.com/ome/openmicroscopy/pull/6349, https://github.com/ome/bioformats/pull/4026, https://github.com/ome/omero-web/pull/482 and https://github.com/ome/omero-py/pull/375