Bump to v0.5.0

[CasperWA]

Since the API is changed (very slightly), we should actually up the major version, however, since the package/repository is still under development - and we are not extremely compliant with SemVer, I think it's okay to bump "just" the minor version instead.

Changes:

• Possibility for Docker deployment for both the index meta-database server as well as the regular server (Add docker for index meta-database #140, @ltalirz, @CasperWA)
• Test building and starting Docker images with GitHub Actions CI (Add docker for index meta-database #140, @CasperWA, @ml-evs, @ltalirz)
• Remove /index from the index meta-database's base URL (Add docker for index meta-database #140, @ltalirz, @CasperWA)
• include query parameter (Implement include query parameter #163, @CasperWA)
• Rename optimade/server/deps.py to optimade/server/query_params.py (Implement include query parameter #163, @CasperWA)
• Human-readable landing page for versioned base URLs, as well as for /optimade (Add landing page at base URL #172, @ml-evs)
• Move mapper aliases to config file and out of mapper classes (Make mapper aliases configurable #175, @ml-evs)

Bug fixes:

• Properly build versioned base URLs (Correctly create optional versioned base URLs #178, @CasperWA)

[codecov]

Codecov Report

Merging #179 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #179   +/-   ##
=======================================
  Coverage   86.31%   86.31%           
=======================================
  Files          40       40           
  Lines        1878     1878           
=======================================
  Hits         1621     1621           
  Misses        257      257

Flag	Coverage Δ
#unittests	86.31% <ø> (ø)	⬆️

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[ml-evs]

My understanding of semver is that it stipulates that you can do what you want to the API before v1 anyway, so think we're still in the clear!