Add operation for creating database indexes and class based indexes

[Akshesh Doshi]

With all the discussion starting from Marc's ​DEP and continuing at various ​other ​places, the first step we can take towards having class based indexes is to create the base Index class and an operation (CreateIndex) to create them. This would allow us to create indexes from the migrations using a defined API of the Index class without even touching the model layer.
A DropIndex will also be required to be created.

[Akshesh Doshi]

A working model - ​https://github.com/django/django/pull/6726/

Misses docs for the new Index class.

[Akshesh Doshi]

Added docs for the new index classes.

[Tim Graham <timograham@…>]

In 156e2d59:

Fixed #26709 -- Added class-based indexes.

Added the AddIndex and RemoveIndex operations to use them in migrations.

Thanks markush, mjtamlyn, timgraham, and charettes for review and advice.

[Tim Graham <timograham@…>]

In b1e7d19d:

Refs #26709 -- Required a name for Indexes passed to AddIndex.

Thanks to Markush for discussions.

[Tim Graham <timograham@…>]

In f7e9071:

Refs #26709 -- Added IndexOperation to avoid code duplication.

[Tim Graham <timograham@…>]

In 52442898:

Refs #26709 -- Added 'model' argument to SchemaEditor.add/remove_index()

This removes the dependency of the Index class on its model attribute
when a name is passed to it.

Thanks to Markush for discussions.

[Tim Graham <timograham@…>]

In 3551fb53:

Refs #26709 -- Documented SchemaEditor.add/remove_index().

[Tim Graham <timograham@…>]

In fc3ac65:

Refs #26709 -- Checked allow_migrate_model() in Add/RemoveIndex operations.

[Tim Graham <timograham@…>]

In b92c6b7:

Refs #26709 -- Replaced Index.get_name() with set_name_with_model().

This removes the dependency of the Index class on its 'model'
attribute.

[Tim Graham <timograham@…>]

In a71724cd:

Refs #26709 -- Added index name to AddIndex.describe().

[Tim Graham <timograham@…>]

In c969b17:

Refs #26709 -- Added type check for models.Index fields argument.

[Markus Holtermann <info@…>]

In e7033e0:

Used constant instead of hard-coded value for max index name length

Refs #26709

[Markus Holtermann <info@…>]

In c654ead8:

[1.11.x] Used constant instead of hard-coded value for max index name length

Refs #26709

Backport of e7033e00f8e1ba2ffe538e56c5088a7e94c2e45d from master

[Mariusz Felisiak <felisiak.mariusz@…>]

In 4c62cdaa:

Refs #26709 -- Made Index raise ValueError on non-string fields.