Handle hashtags in spoiler_texts (partial fix for #699) (#2683)

* services: scan spoiler_text for hashtags (#699)

* views: link hashtags from spoiler_texts

This covers linking hashtags from within the spoiler
text on the server-generated pages.

* services: fix string concat going into hashtag RE

Cleaner Ruby syntax, may handle immutable strings better

app/services/process_hashtags_service.rb

@@ -2,7 +2,8 @@
 
 class ProcessHashtagsService < BaseService
   def call(status, tags = [])
-    tags = status.text.scan(Tag::HASHTAG_RE).map(&:first) if status.local?
+    text = [status.text, status.spoiler_text].reject(&:empty?).join(' ')
+    tags = text.scan(Tag::HASHTAG_RE).map(&:first) if status.local?
 
     tags.map { |str| str.mb_chars.downcase }.uniq(&:to_s).each do |tag|
       status.tags << Tag.where(name: tag).first_or_initialize(name: tag)