feature: standardize how metadata is stored everywhere

[elboletaire]

Describe the feature

Right now we have (at least) two ways of storing metadata:

• In a almost-free way in elections, allowing us to define the meta as we wish
• In the type of Array<{key: string, value: any}> for Account.meta

There's no way to limit how account.meta is stored, like we're doing right now; it would be better to have the same type we used for election.meta.

Motivation

Standardization and ease of use.

Proposal

To use the exact same type signature we used for ElectionMeta in 325aa9f

Components affected

Mainly Account, but I'm not sure right now if we have any other meta field a part from the election and the account.. if we have, it may be a good idea to standardize all of them.