Upgrade to OpenLayers v7

[spwoodcock]

Fixes #329. I upgraded from ol v6.15.7 --> 7.2.0.

There were some minor deprecations to replace:

• Overlay no longer accepts autoPanAnimation as an option. Now the animation delay is passed as an object directly to the autoPan option.
• map.forEachLayerAtPixel() was removed. I replaced it with a map.getLayers().forEach(). This shouldn't be much of a performance issue as getLayers was called internally by forEachLayerAtPixel anyway. Perhaps there is a way to optimise, but I couldn't find it.
• I also updated a Vuetify deprecation that was giving a warning: v-content was simply renamed v-main.

Additional considerations:

• Internet explorer is now no longer supported by ol. I hope that's not a problem.
• With ol v7, now we can work on Point (De-)Cluster Renderer #198 and cog geotiff support on wegue #290.

[spwoodcock]

Superseded by #335.

[spwoodcock]

Merged master to only show specific commits - ready to test and merge.
Please add the Wegue V2 label.
I'm happy to rebase to make the history clearer - let me know.

[spwoodcock]

Closed in favour of #335