Clearer distinction in options/app configuraiton

[PragTob]

Follow up from #506

I found the organization of feature flags as they exist in the application right now a bit hard to follow and mixed up:

• naming doesn't seem to be consistent
• some feature flags are actually more like configuration options

For reference their current state:

# enable/disable brainstorming teasers and configure delete brainstormings option:
config :mindwendel, :options,
  feature_brainstorming_teasers: EnvHelper.enabled?("MW_FEATURE_BRAINSTORMING_TEASER", "true"),
  feature_file_upload: feature_file_upload,
  feature_brainstorming_removal_after_days: delete_brainstormings_after_days,
  feature_privacy_imprint_enabled: feature_privacy_imprint_enabled,
  # use a strict csp everywhere except in development. we need to relax the setting a bit for webpack
  csp_relax: config_env() == :dev

• most of them are pre-fixed feature
• one has an _enabled suffix
• brainstorming removal after days is more of an option

I'd suggest to split it:

• feature flags get their own key to be organized under (either inside of options or another one) to promote organization/consistency and so we can drop the consistent feature_ prefix
• brainstorming_removal_after_days (and csp_relax) can stay in options - we could create an Options module similar to the feature flag module to unify access to them

of course that's a small/cosmetic kind of change but it'd help before it grows more :)