do not automatically show media previews of nsfw posts

[e-five256]

• do not automatically show media previews of nsfw entry / post / entry comment / post comment
• rewrite templates to use set and use defaults on cookie getters rather than null check

the majority of this change is: 'show-preview': SHOW_PREVIEW is same as 'true' and not [entry|post|comment].isAdult
someone mentioned that they wished to see this change once, and I agree with it, you can still preview NSFW media manually, but this doesn't automatically open it on you

I guess perhaps it could also be a user setting, if people do want to browse nsfw mags for instance perhaps they want all to open automatically, this has always been an issue I have had where I feel like there's complete consensus of NSFW vs NSFL yet it always never is implemented (small rant)

the rest is I updated the four components to use set vars similar to what @asdfzdfj did for the subscription sidebar. That looked far more readable to me instead of long constants in the middle of the component. I also added defaults for each setting, so for example SHOW_MAGAZINE_ICONS defaults to true if not set so the code doesn't need to do the first half of what it was: not SHOW_MAGAZINE_ICONS or SHOW MAGAZINE_ICONS is true

some examples if they make sense, this is just me opening the page with show media preview on for both entries and posts, not clicking anything

• entry
  
• entry comments
  
• post and post comments
  

[e-five256]

need to double check this after #301 (edit: appears to work fine)

also feeling less certain, after I opened the PR I edited the comment in about user setting, perhaps worth getting more feedback from people on if this would ruin their experience for browsing nsfw mags

the problem I'm having is I want to add dozens of user and admin settings but each of those being a column seems like not a great table structure. that might just be me, my view on table management is less columns and no nullable fields, if it's optional it should be in another table as not null and FK'd instead, but that's just one way of doing things

[e-five256]

since it didn't seem like there were strong opinions on this, just going to mark it as ready, easy enough to revert if people end up having problems with it or we want to come back and add an additional user setting to allowing nsfw auto preview