Add functionality to get contributors

[oruebel]

• [X] Added GitCodeStats.get_contributors to get the contributors for a single repo from the git log
• [X] Added GitCodeStats.merge_contributors to compile info of all contributors across multiple repos and merge duplicates
• [X] Added logic to detect and remove duplicates based on name and email in GitCodeStats.merge_contributors
• [X] Update GitCodeStats.compute_code_stats logic to compute the contributors and cache the result as a tsv in /data/contributors.tsv. In this way, this is automatically part of the create_codestat_pages workflow.
• [x] Update create_codestat_pages to link to contributors on the NWB summary page and summarize contributors as a list

[oruebel]

This was much more involved to get working than I expected, but it is finally working. This PR adds functionality to automatically compute a list of contributors based on the git log from all the NWB repos. Here an example contributors.tsv generated by the PR. In terms of the logic:

• Contributions are indicated in number of commits
• Users are de-duplicated based on email and name, i.e., if either email or name are matching than we assume the same user. With the information we have from git logs (i.e., without getting user data from GitHub), this is the best we can do.
• This will run automatically and be updated nightly together with the code statistic logs