Add docs for AS::Notifications::Event#duration

[ci skip]
Fix docs for AS::Notifications::Event#duration

Stems from comments by @zzak on e07f3dd:

* https://github.com/rails/docrails/commit/e07f3ddcac394d2a8dc23fc571318b7e8c2497b1#commitcomment-9015634
* https://github.com/rails/docrails/commit/e07f3ddcac394d2a8dc23fc571318b7e8c2497b1#commitcomment-9015639

[ci skip]
Add docs for Time#find_zone

Also improves docs for `Time#find_zone!`

[ci skip]
Add docs for Numeric#*_bytes methods

Add docs for `kilobytes`, `megabytes`, `gigabytes`, `terabytes`,
`petabytes` and `exabytes`. Fix docs for `bytes`.

[ci skip]
Replace `#=>` with `# =>` [ci skip]

@rafaelfranca suggested in f7c7bcd9 that code examples should display
the result after  `# =>` and not after `#=>`.

This commit replaces *all* the occurrences of `#=>` in the code documentation
(mostly added by me :sob:) with the suggested `# =>`.
Add docs for Numeric time-related methods

Add docs for `minutes`, `hours`, `days`, `weeks` and `fortnights`.
Fix docs for `in_milliseconds`.

[ci skip]
`db:structure:load` and `db:schema:load` no longer purge the database.

Closes #17945

`db:test:prepare` still purges the database to always keep the test
database in a consistent state.

This patch introduces new problems with `db:schema:load`. Prior
to the introduction of foreign-keys, we could run this file against
a non-empty database. Since every `create_table` containted the
`force: true` option, this would recreate tables when loading the schema.

However with foreign-keys in place, `force: true` wont work anymore and
the task will crash.

/cc @schneems
Spelling fix [ci skip]
Merge pull request #18087 from georgemillo/patch-2

Spelling fix [ci skip]
Add docs for AS::TestCase::test_order

Document `test_order` and `test_order=` from `ActiveSupport::TestCase`.

[ci skip]
Merge pull request #18077 from claudiob/add-docs-for-test-order

Add docs for AS::TestCase::test_order
Enable Travis CI container-mode

And also bundler cache
Add docs for ActionDispatch::Http::URL methods

Add docs for `extract_domain`, `extract_subdomains`, `extract_subdomain`.

Add doc examples for `url`, `protocol`, `raw_host_with_port`, `host`,
`host_with_port`, `port`, `standard_port`, `standard_port?`, `optional_port`,
`port_string`.

[ci skip]
Output extension is no longer generated [ci skip]

/cc @senny Is this change documented somewhere?
Clarity start parameter

I find that `Specifies the starting point for the batch processing.`
does not give enough information for me to understand what this
parameter actually does.
Revert "Merge pull request #18003 from sikachu/permit_all_parameters-thread-safety"

This reverts commit da5cc10e945552da54234f858470238a3fc36767.

Fixes #18091

See also https://github.com/rails/rails/pull/18003#commitcomment-9030909
Use released rack

`rack.version` was bumped in https://github.com/rack/rack/commit/28e77c710ac2ddb35c436d78d72e28f28f964446
`force: :cascade` to recreate tables referenced by foreign-keys.
Merge pull request #18082 from rails/17945_purge_in_test_only

`db:schema:load` and `db:structure:load` do not purge the database
Use released I18n 0.7.0