rss_style: include guid in tickets RSS

SlapOS ticket RSS have one entry for each ticket, with the content of
the latest event as description, but when using a feed reader application,
we'd like to have a new entry for each message posted to the ticket, to
be notified that a new message was posted.

Feed reader applications fetch the RSS and have to determine if each
entry is already in their database or not. In a scenario like below, we
observe different behaviors for different applications:

  1. a user open a ticket "help" with a first message "I need help !".
    The RSS entry for this ticket is:

    <item>
      <link>https://slapos.example.com/#/support_request_module/1</link>
      <title>Help</title>
      <pubDate>Mon, 01 Jan 2001 10:57:10 +0100</pubDate>
      <description>I need help !</description>
    </item>

   => feed reader fetches the RSS and add a new entry with title "Help"

  2. support operator reply with a second message "how can I help you ?"
    The RSS entry for the ticket becomes:

    <item>
      <link>https://slapos.example.com/#/support_request_module/1</link>
      <title>Help</title>
      <pubDate>Mon, 02 Jan 2001 08:32:12 +0100</pubDate>      <== this is different
      <description>How can I help you ?</description>           <== this is different
    </item>

now depending on the implementation of the feed reader, it will either:

 - add a new entry, this is the best behavior. In the RSS readers I tested,
  liferea and newsboat do this. Probably they consider that the entry is
   different because it has a new pubDate and/or description.
 - update the existing entry "in place" - this is not so good, because
  this does not appear as a new entry and the original message is no
  longer in the RSS reader. TinyTinyRSS behaves like this (at least in
  version 17.4, which is a bit old)
 - consider they already have the entry and don't do anything - this is
  even worse, because user can not see there was a reply. FreshRSS
  behaves like this.

A RSS for tickets will always have the problem that the feed reader must
refresh often enough if we want to have a copy of all messages in the
feed reader (and that's why ERP5 CRM's uses a RSS of Events and not
Tickets), but we can make this situation better by using guid on messages.

By constructing a guid [1] that will become different every time a new
message was posted, feed readers consider each item as a a new, different
entry. At least this is the case for all the feed reader I tested, except
ERP5 builtin feed reader which only consider the `link` attribute.

Strictly speaking this implementation has a "problem" that these guids
are not permalinks, but I believe that's something we should address in
erp5_rss_style (if guid does not look like an URL, set isPermalink="false"),
but in practice it does not seem to be a problem.

1: https://validator.w3.org/feed/docs/rss2.html#ltguidgtSubelementOfLtitemgt