changed error code for access to deleted post

See merge request !294

[ABW]: changed error code for access to deleted post (previously used code was mistakenly taken from reserved range)

Custom error for access to deleted post

See merge request !290

[ABW]: added custom error triggered by find_comment_id() to make difference between deleted post and nonexisting one
all API calls that use this SQL function are affected (unfortunately it is not yet used everywhere)
get_content/get_content_replies reimplemented as SQL functions
removed superfluous code from condenser_api
restored mute handling in get_content_replies for consistency with get_content

Changes additionally introduced in deployed20200928 branches, which shall be...

See merge request !287

[JES] Pull blacklists changes back in to current version of develop.

See merge request !275

 Conflicts:
	tests/tests_api

Changes additionally introduced in deployed20200928 branches, which shall be merged back to develop.

fix notification index bug and add tests

See merge request !286

Merge branch 'jes2850-blacklists-take-2' of gitlab.syncad.com:hive/hivemind into jes2850-blacklists-take-2

Optimization of notification_apis:

See merge request !282

- introduced reduced notification view, providing raw account and post IDs to eliminate joins to big tables before actual filtering
- updated notification API functions to use faster view.

Minor fixes in list_comments and get_ranked_posts

See merge request !281

[ABW]: relaxed checks in list_comments with 'by_permlink' order - since those are basically treated as strings it was inconsistent to enforce size and chars
fixed missing order and filtering for deleted posts in bridge_get_ranked_post_by_trends_for_observer_communities (problem introduced during recent optimization)

`get_content` filled missing fields

See merge request !257