Peppol xml front end changes

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The front-end changes for confac. User is able to download the PEPPOL xml on the edit invoice-page and can send the xml as an attachment via e-mail.

The saving of a client has been changed: instead of country name, the ISO 3166 country code is saved in the field 'country'. This data is needed to comply with the PEPPOL standard. The user interface still shows the old country names. The exisiting data in the MongoDB database will need to be updated too.

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Scripts to change the Client data in the MongoDB:
db.getCollection('clients').find({country:'België'}).forEach(function(doc) {
db.getCollection('clients').updateMany({ _id: doc._id },{ $set: { country: 'BE'}});
})
db.getCollection('clients').find({country:'Nederland'}).forEach(function(doc) {
db.getCollection('clients').updateMany({ _id: doc._id },{ $set: { country: 'NL'}});
})
db.getCollection('clients').find({country:'UK'}).forEach(function(doc) {
db.getCollection('clients').updateMany({ _id: doc._id },{ $set: { country: 'GB'}});
})
db.getCollection('clients').find({country:'Duitsland'}).forEach(function(doc) {
db.getCollection('clients').updateMany({ _id: doc._id },{ $set: { country: 'DE'}});
})
db.getCollection('clients').find({country:'Frankrijk'}).forEach(function(doc) {
db.getCollection('clients').updateMany({ _id: doc._id },{ $set: { country: 'FR'}});
})