Add vaadin:upgrade8 which runs the migration tool

[ahie]

This change is 

[CLAassistant]

All committers have signed the CLA.

[hesara]

Reviewed 1 of 2 files at r1, 1 of 1 files at r2.
Review status: all files reviewed at latest revision, 1 unresolved discussion.

---

pom.xml, line 401 at r1 (raw file):

    </dependency>
    <dependency>
      <groupId>com.vaadin</groupId>

(there is a chance that the aether dependencies of the migration tool conflict with those of the Maven plug-in at some point - not sure if we can do much about it now, but we might need to prepare to not have it as a plug-in dependency but download the JAR with aether and run it in a separately forked VM in some future version of the plug-in)

---

Comments from Reviewable