Steps to reproduce:
 - Install the planning module.
 - Open Gantt view in day scale.
 - Create a planning slot from 11am to 12pm for an employee whose working hours
   are 8-12 and 13-17
 - The default hours will be shown as 13 to 12

Issue:
 - When an employee works two shifts for example 8-12 and 13-17, and we are
   creating a slot from 11-12, the slot timing will show as 13-12 instead of
   8-12.

Cause:
- here is an example of how the planning slot will be calculated and what is
   miscalculation
 - When click on cell 9-10 the calculated hours
   <table><thead><tr><th>Cell</th><th>shift start time </th><th>shift end time </th></tr></thead><tbody><tr><td>9-10</td><td>|8 - 9| = 1<br>|13 - 9| = 4<br></td><td>|12 - 10| = 2<br>|17 - 10| = 5<br></td></tr><tr><td>minimum</td><td>8</td><td>12</td></tr></tbody></table>
 

   so that slot timining will be 8 - 12

- When click on cell 11-12
  <table><tbody><tr><td>Cell</td><td>shift start time </td><td>shift end time </td></tr><tr><td>11-12</td><td>|8 - 11| = 3<br>|13 - 11| = 2<br></td><td>|12 - 12| = 0<br>|17 - 12| = 5<br></td></tr><tr><td>minimum</td><td>13</td><td>12</td></tr></tbody></table>
   So that the slot timining will be 13-12 which should not be possible

Fix:
- We previously relied on the ```_adjust_to_calendar``` method to calculate the
  resource's working hours. However since the working hour intervals are
  already computed using ```_work_intervals_batch```, we are removing the method
  call to reduce code duplication. Calculate the working hours using
  ```_work_intervals_batch``` and return the appropriate work intervals based on
  the resource's working calendar.

task-3916687

Forward-Port-Of: odoo/enterprise#64961

Some users need to make invoices with 100% discount lines. If they try then will receive this UCFE error that does not let us to validate the invoice in DGI .

> ERROR: CODE 31: Error: El contenido del elemento 'Item' en espacio de nombres 'http://cfe.dgi.gub.uy' está incompleto. Lista esperada de elementos posibles: 'SubDescuento, RecargoPct, RecargoMnt, SubRecargo, RetencPercep, MontoItem' en espacio de nombres 'http://cfe.dgi.gub.uy'. 

In order to make it work we need to adapt the module to accept this kind of lines that actually represent an "Entrega Gratuita" (Indicator 5 in Uruguay)

- We check if the line is a 100% discount, if it is then we marked as Entrega Gratuita when we are creating XML (Indicator 5)
- Now that we can have lines and also invoices with total value 0.0 we need to adapt our XML because if value 0.0 then some tags were not added to the final XML - it was interpreted as False and the result was and invalid XML becasue we were missing a mandatory tag. Now we can tags that have 0.0 value to the final XML.
- Now that we can have lines 0.0 total amount, and also invoices 0.0 total amount, if we are using foreign currency then we need to avoid error computing the currency rate (division by zero). Now we detect this cases and use the date rate. Also, the rate is only been computed when actually we are going to use it.

LATAM 1296 / ADHOC Ticket 83295