Cleanup LICENSE files. (docker-mailserver#47)

* Cleanup LICENSE files.

Retroactively create LICENSE at root of repository.  Set copyright year to
date of first commit in repo, and owner to davidy, who made most of the
commits.  Add second entry for collective authors based on first year of
other authors.

Delete LICENSE inside chart directory for two reasons:
1- it doesn't appear to line up with actual authorship of the helm chart.
2- GitHub will surface it better at the root.

* Bump chart version.

Co-authored-by: David Young <[email protected]>

LICENSE

@@ -0,0 +1,22 @@
+The MIT License (MIT)
+
+Copyright 2019 David Young
+Copyright 2020 The Docker Mailserver Helm Chart Authors
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.