Update glitch-soc-specific dependencies

- drop html2text which has been unused in a while
- update redcarpet

Gemfile.lock

@@ -270,8 +270,6 @@ GEM
     highline (2.0.3)
     hiredis (0.6.3)
     hkdf (0.3.0)
-    html2text (0.2.1)
-      nokogiri (~> 1.6)
     htmlentities (4.3.4)
     http (4.3.0)
       addressable (~> 2.3)
@@ -493,7 +491,7 @@ GEM
       link_header (~> 0.0, >= 0.0.8)
     rdf-normalize (0.4.0)
       rdf (~> 3.1)
-    redcarpet (3.4.0)
+    redcarpet (3.5.0)
     redis (4.1.3)
     redis-actionpack (5.0.2)
       actionpack (>= 4.0, < 6)
@@ -706,7 +704,6 @@ DEPENDENCIES
   hamlit-rails (~> 0.2)
   health_check!
   hiredis (~> 0.6)
-  html2text
   htmlentities (~> 4.3)
   http (~> 4.3)
   http_accept_language (~> 2.1)