Apply naming conventions to constraints and indexes

Problem

Although there are no specific conventions that we should follow at GitLab for naming database indexes and constraints, we should adopt one to improve consistency and discoverability.

Solution

Use the following naming convention:

Type	Syntax	Example
PRIMARY KEY constraints	pk_<table name>	pk_repositories
FOREIGN KEY constraints	fk_<table name>_<column name>_<referred table name>	fk_repository_manifests_manifest_id_manifests
UNIQUE constraints	uq_<table name>_<column(s) name>	uq_manifest_layers_manifest_id_layer_id
CHECK constraints	ck_<table name>_<column(s) name>_<validation name>	ck_layers_media_type_length
Indexes	ix_<table name>_<column(s) name>	ix_tags_manifest_id

This was inspired in https://alembic.sqlalchemy.org/en/latest/naming.html.

Related to &2313 (closed).