index auto-name collisions

## What did you do?

If you take this table

```sql
CREATE TABLE "public"."test" (
"id" serial,
"flag1" bool NOT NULL DEFAULT 'false',
"flag2" bool NOT NULL DEFAULT 'false',
PRIMARY KEY ("id")
);
```

and use the structure view to create two expression indices like `flag1 = true` and `flag2 = true`. The auto generated names for the indices are both `test_expr_idx`.

## What did you expect to happen?

The index names should be checked for collision with existing indices and the other ones that will be created.

Or the index name should not be specified in the create query (if not explicitly specified by the user) so that postgres takes care of naming them.

## What actually happened?

Save changes fails with `PostgreSQL said: relation "test_expr_idx" already exists`;

## What software versions are you using?

Postico version: 1.3.5
macOS version: 10.13.4
PostgreSQL version: 10.3

Reproduce the collision from the Structure view using the reported table and two expression indices, then trace the index-name generation used when saving changes. Done means separately named indices can be created successfully, or unnamed indices are left for PostgreSQL to name, without the relation-already-exists error.

Written by the indexing model from the issue text.