Move cache and session store to Redis

This MR moves two components to Redis.

1. Caching — before, we were relying on memcached for caching. This is unnecessary, because we alrady mandate Redis as message broker, and Redis is as performant as memcached for caching as well.
2. Sessions — as we mandate Redis anyway, storing sessions there is possible as well. This is the last missing piece to support horizontal scaling of AlekSIS.

added part::backend type::feature labels

added 2 commits

• 94d48989 - Replace memcached with redis in docs
• f285e578 - Remove memcached dependency

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added 4 commits

• 2f7839ee - Use Redis as cache backend by default
• 461e79e0 - Use Redis as session store
• 5166412e - Replace memcached with redis in docs
• 91cce60b - Remove memcached dependency

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added 1 commit

• 31c8605f - Disable Redis under test suite

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added 1 commit

• 82f5f0b8 - Disable Redis under test suite

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marked this merge request as draft

added 2 commits

• b2b3937b - Use Redis as session store
• adfbad45 - Replace memcached with redis in docs

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marked this merge request as ready

added 1 commit

• e2829a44 - Ignore caching exceptions if Redis is unavailable

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added 12 commits

• e2829a44...cf471364 - 11 commits from branch master
• 8fd72a43 - Merge branch 'master' into feature/redis-cache-and-sessions

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merged

mentioned in commit 86c15195