Campaign Manager 1.3-beta1

Changes since Campaign Manager 1.2

• [#84] Replace image icons with FontAwesome icons - by @pfigel
• [#87] MkDocs upgrade fix - by @homotechsual
• [#88] More MkDocs upgrade fixes - by @homotechsual
• [#90] array_key_exists() parameter order - by @MarcMichalsky
• [#91] Add buttons to export KPI data - by @bjendres, @BenediktMagnus, @jensschuppe
• [#93] Improve pie chart for KPI visualisation - by @BenediktMagnus, @jensschuppe
• [#94] Improve Donation Heartbeat chart for KPI visualisation - by @jensschuppe
• [#96] Correctly link contact custom fields in campaign view - by @sebalis, @zfranque
• [#97] Link to contribution detail report from KPIs - by @jensschuppe
• [#99] Rebuild civix code for PHP 8 support (and avoid deprecated warnings in PHP 7.4) - by @IProSoft, @jensschuppe
• [#100] Position of download buttons shifted - by @abugey
• [#101] Graph not showing correctly in Firefox - by @abugey
• [#103] PHP 8 compatibility - by @abugey
• [#105] Fix no campaigns displayed on PHP8 site - by @olayiwola-compucorp
• Make donation heartbeat SQL query compatible with sql_mode ONLY_FULL_GROUP_BY - by @jensschuppe
• Division by zero mitigation - by @bjendres