Show space usage for materialized views

[fatkodima]

Currently, the "Space" section contains space info only about tables and indexes, but not materialized views, even though that indexes for materialized views are already shown. This PR fixes that.

I have a few other improvement ideas to this gem:

1. Add some info about indexes on columns having a very high NULL fraction (pg_stats.null_frac), or indexes on columns with a high % of the same value (pg_stats.most_common_freqs), or indexes on boolean columns? These indexes should be made partial or removed.
2. Show also space occupied by partitioned tables.
3. Add to the "Space" section for tables a column showing its size with all its indexes. Or this can be a clickable link from the "Space" section for tables that show a new view with this info and some other details

Can help with PRs.

[ankane]

Thanks @fatkodima! Happy to include 2 as well.