Merge neurons

[anishh2003]

Motivation

The governance canister supports merging neurons provided that they are controlled by the same principal.
But currently there is no way to use this functionality via the NNS frontend Dapp.
This PR makes it easy for users to merge neurons by adding a new 'Merge Neurons' button to the neurons tab.

Changes

• Added merge to ServiceApi.ts which calls manage_neuron on the governance canister using the merge command
• Give users the option to merge neurons via a new 'Merge Neurons' button on the neurons tab

[hpeebles]

When a user requests to merge 2 neurons, the only check we do in the browser is that the 2 neurons are controlled by the same principal.
There are additional restrictions (eg. neither neuron can be in the community fund and the 2 neurons must have matching followees on the 'ManageNeuron' topic), but these are only checked in the governance canister since it seems possible that these restrictions will be dropped in the future.
Any errors which the governance canister returns are shown to the user so if any of the checks fail the user will be told the reason.

[hpeebles]

This code is deployed to here - https://qvhpv-4qaaa-aaaaa-aaagq-cai.nnsdapp.dfinity.network/v2/
I have tested the various scenarios and everything looks to be working.
The target and source neurons are always assigned correctly so as to ensure age bonus is always preserved.
The error messages bubble up to the end user whenever they are unable to merge neurons (eg. the controllers don't match).