Code-only improvement: avoiding reimplementing the built in truncate method

[trichoplax]

Describe the bug
In the file app/views/users/_activity_items.html.erb there are 4 occurrences of truncating a string using a line of code, which has identical behaviour to the built in truncate method:

• line 22: <%= i.body_plain[0..300] + ((i.body_plain.length > 300) ? "..." : "") %><br>
• line 34: <%= i.content[0..300] + ((i.content.length > 300) ? "..." : "") %><br>
• line 73: <%= i.body[0..300] + ((i.body.length > 300) ? "..." : "") %>
• line 88: <%= i.reason[0..300] + ((i.reason.length > 300) ? "..." : "") %><br>

For example usage of the truncate method, see app/views/posts/show.html.erb line 1.

We could avoid this duplicated code by replacing these 4 occurrences with calls to truncate.