Housekeeping: unused dependencies, single-line inputs, own links

[Yarroo]

Follow-up to #1, in three groups.

Snapshot correctness and cost

• fix a race between snapshot rebuild and invalidation — the rebuild assigned @snapshot and then read the ivar back, while reset_snapshot cleared it without the mutex. An after_commit landing between those two lines gave nil.values. The same gap also lost invalidations: a rebuild that started before a commit would finish afterwards and store pre-commit values under a fresh timestamp, so the process that saved a setting served the old value for a whole TTL.
• serve a stale snapshot while another thread rebuilds it — the rebuild runs a query while holding the mutex, so every other thread in the process used to queue behind it. Now a stale snapshot is served immediately and only one thread rebuilds; a cold start still waits, there is nothing to serve.
• build the snapshot from an unscoped relation — all honours current_scope, so a call made inside relation.scoping would have cached a truncated table for the whole TTL.
• memoize the settings config instead of re-reading yaml on every call — load_config read the file, ran ERB and parsed YAML on every call, and #value asks for all_settings two or three times. Building a snapshot of a thousand settings meant thousands of parses, under the mutex. reload_config! clears both.

Rails compatibility

• replace render :text and update_attributes — removed in Rails 5.1 and 6 respectively; AdminUsersController could not work at all on a current Rails.
• keep plain settings on a single-line input — the migration already creates t.text :string so long values are not truncated, which makes formtastic infer a textarea for every plain setting. This is the other half of Fix long strings, prevent 255 char truncation slate-studio/activeadmin-settings#25.

Housekeeping for a standalone repository

• drop unused runtime dependencies — the uploaders went away in f8146e0 and carrierwave, mini_magick and devise are not referenced anywhere since. Version goes to 0.6.0.
• drop mongoid instructions — the model is ActiveRecord::Base; those sections only kept the removed dependencies alive in the docs.
• point homepage and readme images at this repository. Authorship left as it was.

Verified on a live Rails app: 60 reads from a cold snapshot cost 1 query and the next 60 cost none; with the mutex held elsewhere a read of a stale snapshot returns in 0.1 ms instead of blocking; 3200 reads across 8 threads alongside 60 writes raise nothing; a value read after a write is fresh.