<summary> in Atom feed does not escape "&" as "&amp;", broken XML entity

Environment

• Installation type (OTP or From Source): From Source
• Pleroma version (could be found in the "Version" tab of settings in Pleroma-FE): 2.1.2 (with Pleroma-FE of b225c357)
• Elixir version (elixir -v for from source installations, N/A for OTP):
  • Erlang/OTP 23 [erts-11.1.1] [source] [64-bit] [smp:2:2] [ds:2:2:10] [async-threads:1]
  • Elixir 1.10.3 (compiled with Erlang/OTP 22)
• Operating system: Debian bullseye
• PostgreSQL version (psql -V): psql (PostgreSQL) 13.0 (Debian 13.0-4)

Bug description

Pleroma does not escape & as & within the <summary> tag on the Atom feed for a post with & in the Subject/Content Warning.

Steps to reproduce

1. Post a public, listed status with a subject of "A & B"
2. Fetch the Atom feed for this user
   • E.g. https://example.com/users/ExampleUser/feed.atom
3. Try to parse it with an XML parser (tested with PHP's libxml and Firefox's XML viewer with RSS Preview)

Expected results

The & gets escaped to & within the <summary> tag, e.g.

<summary>A &amp; B</summary>

Actual results

The & is passed on without escaping within the <summary> tag, breaking XML parsing.

<summary>A & B</summary>

PHP's LibXML says:

LibXML error 68 at line ### (column 40): xmlParseEntityRef: no name

Firefox with the RSS Preview extension says:

XML Parsing Error: not well-formed
Location: https://example.com/users/ExampleUser/feed.atom
Line Number ###, Column 40:

Possible fix

Make use of the escape() function referenced in _author.atom.eex…

Modifications to _activity.atom.eex:

   <%= if @data["summary"] do %>
-    <summary><%= @data["summary"] %></summary>
+    <summary><%= escape(@data["summary"]) %></summary>
   <% end %>

Note: Other areas might need escaping applied as well, e.g. the RSS feed templates, which also seemed unable to be parsed at this time (non-namespaced prefixes).