Fix migrations with enable_extension

When using ActiveRecord::Base.table_name_prefix and/or
table_name_suffix, extension names got the same treatment as table names
when running migrations. This led to migrations that tried to call, for
example, enable_extension("prefix_hstore") on the connection.
Reorganize ActiveRecord tasks [Arun Agrawal & Abd ar-Rahman Hamidi]
PostgreSQL timestamps are always datetimes

The current behavior is that they are treated as `datetime` normally,
but if they are part of an array, they are treated as `timestamp`. The
only place that seems to be impacted by this is schema dumping, which
shouldn't matter since `t.datetime` and `t.timestamp` are equivalent in
the `PostgreSQL` adapter, anyway.
Use destructured arguments when looping through pairs

Minor refactoring of looping behavior for aggregation
Default config.assets.digests to true in development
Make `:index` in migrations work with all column types
early return on delete and destroy methods

When delete or destroy is called on all records nothing
is deleted or destroyed. Intead of running through the code and still
not deleteing anything, we should early return
Let others know why this code is here

[ci skip]
remove ecosystem link that is broken
Merge pull request #15169 from Gaurav2728/gaurav-ecosystem

remove ecosystem link that is broken [ci skip]
Merge pull request #15157 from msgehard/remove_cruft_in_has_secure_password

Remove cruft now that strong_parameters is default [ci skip]
copy edits[ci skip]
A tiny pass through the PostgreSQL guide [ci skip]
Feature detect based on Ruby version.

I didn't want to do this, FNM_EXTGLOB is defined on 2.1.x, but Dir.glob
returns the wrong value on Ruby less than 2.2.0.  Checking for a
case-insensitive FS seems too hard, so just check Ruby version  Checking
for a case-insensitive FS seems too hard, so just check Ruby version.
Merge pull request #15156 from sgrif/sg-postgres-timestamps

PostgreSQL timestamps should always be datetimes
pg guide, move introductory sentences into main content. [ci skip]

This is a reaction to https://github.com/rails/rails/commit/7ca75f3789e316b6f3dc1f8000ec4f85c8ba32ea#commitcomment-6303828
Merge pull request #14962 from arunagw/aa-fix-rake-activerecord

Reorganize ActiveRecord tasks [Arun Agrawal & Abd ar-Rahman Hamidi]
pg, add missing nodocs for extracted modules.
pg, re-introduce `PostgreSQL::Utils` to unify schema/table extraction.

Partial revert of c0bfc3f412834ffe8327a15ae3a46602cc28e425
Merge pull request #15160 from sgrif/sg-destructured-each

Use destructured arguments when looping through pairs