Get rid of "relativePath" for graylog-plugin-web-parent

[joschi]

Description

Move ./graylog-plugin-web-parent to ./graylog-plugin-parent/graylog-plugin-web-parent to get rid of the relativePath tag in the Maven POM of plugins (and thus the reliance on a specific directory layout).

Motivation and Context

The original author of the plugin parent POMs (hi! 👋) chose the flat directory layout for simplicity but ultimately failed to see the problems which would arise when breaking the Maven conventions.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.
• I have read the CONTRIBUTING document.
• All new and existing tests passed.

[kroepke]

Looks good to me.
In order to make graylog-project work for plugins in the future, we agreed on the following changes for plugins:

• add relativePath to the parent section as appropriate to find the correct parent in a single reactor build
• add Sonatype snapshot repository to each plugin to allow standalone build to succeed (e.g. those on TravisCI running the Java unit tests). Javascript build still requires the local checkout of the server source code.