Replace template placeholders and duplicate code

[bartromgens]

• removes some duplicate template code
• replaces sidebar require placeholders with Django template includes
• replace tabs with spaces in templates
• some minor template code clean-up

[bartromgens]

@mathemaat the commit that replaces tabs with spaces (130e6fd2d4e46f2879146228ae7ad2f2931dc110) makes it difficult to review the whole merge diff at once (sorry!).

Therefore probably easiest to review the commits individually.

Also note that I did not touch blog_post_detail.html because you have changed it already in your branch.

[bartromgens]

Ik heb deze gemerged omdat het handig basiswerk is voor ander werk.

Kan nog steeds een korte review gebruiken hoor :)