Fix: Grenade weaponbox not deploying on unarmed player

[dystopm]

How to reproduce:

• Test environment cvars:
  • mp_buytime -1
  • mp_startmoney 10000
  • mp_forcerespawn 1
  • mp_ct_give_default_knife 0
  • mp_t_give_default_knife 0
  • mp_nadedrops 1
• Join CT, buy hegrenade from buymenu and then change team to T
• You'll drop your hegrenade cause of death, and respawn in a few seconds
• Having no guns (ensure you have none), you'll pickup that hegrenade and see you are not deploying it until you force it (pressing 4, or typing weapon_hegrenade in console) - may lead in a HUD bug in some cases

This commit fixes two issues:

• Grenade weaponbox not deploying on unarmed player after being picked up from ground.
• HEGrenade not being deployed when picked up from ground if you are holding a Flashbang, considering HEGrenade has more weight than Flashbang.

This issue is related to weaponbox and not armoury_entity

TODO: Weaponbox gives ammo AFTER weapon is linked to player, CanDeploy depends on ammo in exhaustible weapons, we can consider this proposal as a final solution to that; or refactor the whole weaponbox behaviour.

Related to #479 but applied in a better fashion way, with explanation