Create and process external MDC events to test RAVEN pipeline; fixes #401 and #403

These changes create and upload external MDC events in order to regularly test the fully RAVEN pipeline. This is designed to create an external MDC event for every 2 MDC superevents, resetting every day. These joint coincidences should be able to be found by ligo-raven>=2.0 by including search='MDC' in ligo.raven.search.search().

The MR includes changes to:

• create MDC external events via first2years_external.py
• use se_search field for superevent searches with new release of ligo-raven
• update superevent with coinc FAR values when better coincidence is available

.gitlab/issue_templates/Create a Release.md

@@ -54,5 +54,17 @@
 16. [ ] Issuing an `ADVOK` signoff through GraceDB results in an initial VOEvent.
 17. [ ] Issuing an `ADVNO` signoff through GraceDB results in a retraction VOEvent.
 18. [ ] Requesting an update alert through the [Flask dashboard](https://emfollow-playground.ligo.caltech.edu/gwcelery) results in an update VOEvent.
+19. [ ] Playground has recently [produced an MDC superevent with an external coincidence](https://gracedb-playground.ligo.org/latest/?query=MDC+EM_COINC&query_type=S), i.e. with a `EM_COINC` label. Use the [Flask dashboard](https://emfollow-playground.ligo.caltech.edu/gwcelery) to do this manually.
+20. [ ] The joint MDC superevent has the following annotations.
+    - [ ] `coincidence_far.json`
+    - [ ] `bayestar-ext.fits.gz` or `subthreshold.bayestar-ext.fits.gz`
+    - [ ] `bayestar-ext.png` or `subthreshold.bayestar-ext.png`
+21. [ ] The joint MDC superevent has the following labels.
+    - [ ] `EM_COINC`
+    - [ ] `RAVEN_ALERT`
+    - [ ] `GCN_PRELIM_SENT`
+22. [ ] The joint MDC superevent is sending alerts with coincidence information.
+    - [ ] At least one VOEvent with `<Group name="External Coincidence">`.
+    - [ ] At least one circular w/ `-emcoinc-` in filename.
 
 /label ~Release