🔧(backend) Allow overwriting the data directory (#893)

## Purpose

Deployments that don't rely on Docker should be given the option to use
a different data directory.

## Proposal

Allow customization of the `DATA_DIR` through an environment variable of
the same name.

If the environment variable is not set the behaviour remains the same as
before.

Signed-off-by: Martin Weinelt <hexa@darmstadt.ccc.de>

CHANGELOG.md

@@ -18,6 +18,7 @@ and this project adheres to
 - 🚩 add homepage feature flag #861
 - ✨(settings) Allow configuring PKCE for the SSO #886
 - 🌐(i18n) activate chinese and spanish languages #884
+- 🔧(backend) allow overwriting the data directory #893
 
 ## Changed